The present invention relates to a bottom-gate-type, thin film transistor, and also to a method for producing it.
Known are thin film transistors (hereinafter referred to as TFT) having an active layer of a silicon film formed on a glass or quartz substrate.
Various types of thin film transistors are known, of which, at present, bottom-gate-type ones are being most industrialized for practical use.
In view of their productivity, it is desirable to develop much in future bottom-gate-type TFT to which the production process in part, the designing rule and the production equipment are common.
A bottom-gate-type, thin film transistor comprises a gate electrode, a gate-insulating film and an active layer of a silicon film as formed on a substrate in that order.
As the silicon film constituting the active layer, generally used is an amorphous silicon film. However, in order to obtain TFT of higher quality, it is desirable to use a crystalline silicon film.
To obtain a crystalline silicon film, much used is a technique of crystallizing an amorphous silicon film through exposure to laser radiations.
For the crystallization, also employable is heating, which, however, does not apply to the production of bottom-gate-type TFT.
This is because, in the process for producing bottom-gate-type TFT, the heating shall follow the formation of the gate electrode, by which the gate electrode material will diffuse unfavorably.
However, from the viewpoint of the quality of the crystalline silicon film produced and of the process of producing the film, heating is preferred to laser irradiation.
As the material of the gate electrode, most preferred is aluminum with low resistance.
However, the electrode material of aluminum is problematic in that it will diffuse, as being influenced by the heating of the active layer for crystallization and activation, to often form projections of so-called hillocks and whiskers.
In particular, in the production of bottom-gate-type TFT, the gate electrode is first formed and thereafter the active layer is formed thereon. In this, therefore, the influence of heat on the layers to be formed in different steps is often problematic.
The subject matter of the present invention disclosed herein is to provide a constitution of a bottom-gate-type TFT having an active layer of a crystalline silicon film.
It is also to provide a novel constitution of a bottom-gate-type TFT having a gate electrode comprising aluminum.
Specifically, the subject matter of the invention is to provide a constitution of a bottom-gate-type TFT having a gate electrode of an aluminum material and having an active layer of a crystalline silicon film.
One aspect of the invention disclosed herein is a semiconductor device, which comprises;
a gate electrode,
a gate-insulating film as formed over said gate electrode, and
an active layer of a crystalline silicon film as formed on said gate-insulating film,
and is characterized in that;
said active layer is so constituted that the crystal growth therein extends from the source and drain regions to the channel-forming region, and
said source and drain regions contain a higher concentration of a metal element capable of promoting the crystallization of silicon than said channel-forming region.
In that constitution, the metal element capable of promoting the crystallization of silicon is most preferably nickel.
Apart from nickel, the metal element may be selected from Fe, Co, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au.
In place of the crystalline silicon film, employable is a compound film of silicon and germanium.
Another aspect of the invention is a method for producing a semiconductor device, comprising;
a step of forming a gate electrode on a substrate,
a step of forming a gate-insulating film on said gate electrode,
a step of forming an amorphous silicon film on said gate-insulating film,
a step of forming a mask on said amorphous silicon film above said gate electrode,
a step of introducing a metal element capable of promoting the crystallization of silicon, into said amorphous silicon film via said mask,
a step of heating the thus-layered substrate to induce crystal growth in said amorphous silicon film in the direction from the region into which said metal element has been introduced to the region below said mask,
a step of doping phosphorus into the region into which said metal element has been introduced, via said mask, and
a step of again heating the substrate to thereby concentrate said metal element in said phosphorus-doped region.
The invention disclosed herein is characterized in that the gate electrode has a laminate structure composed of a thin titanium film and a thick aluminum film, and that the aluminum film constituting the gate electrode is subjected to anodic oxidation.
In the process of producing the device having the constitution noted above, the layered substrate is heated prior to forming the oxide film through anodic oxidation to thereby intentionally make the aluminum film have projections of so-called hillocks and whiskers on its surface. This is to prevent the formation any other projections on the surface of the aluminum film in the subsequent steps.
In the process, in addition, the heating induces the crystallization of the aluminum film due to the action of the titanium film, thereby retarding the formation of hillocks and whiskers on the surface of the aluminum film in the subsequent steps.
Therefore, the constitution noted above is advantageous in that, even when the layered substrate is heated in the subsequent steps, aluminum elements constituting the aluminum film do not diffuse into any other films, or the aluminum film is not melted to flow out anywhere, or hillocks and whiskers are not formed on the surface of the aluminum film.
It is important to side-etch the titanium film and to fill the side-etched region with the aluminum oxide as formed through anodic oxidation. This is because the aluminum diffusion and melt flow to be caused by the heating occurs essentially in the interfacial boundary between the aluminum pattern and the substrate below the pattern.
The constitution of the invention disclosed herein is extremely useful in bottom-gate-type TFT in which the active layer is made of a crystalline silicon film. This is because the process of producing such bottom-gate-type TFT requires a step of forming the crystalline silicon film after the formation of the gate electrode.
In the invention disclosed herein, aluminum can be used to form the gate electrode, and the substrate having an aluminum pattern to be the gate electrode formed thereon can be heated.
Another aspect of the invention disclosed herein is a semiconductor device, which comprises;
a gate electrode of a laminate film composed of a titanium film and an aluminum film,
and is characterized in that;
said titanium film is side-etched via the mask of said aluminum film, and
said aluminum film is subjected to anodic oxidation to form an oxide film on its surface.
Still another aspect of the invention is a semiconductor device provided with a thin film transistor, which comprises;
a gate electrode of a laminate film composed of a titanium film and an aluminum film,
and is characterized in that;
said titanium film is side-etched via the mask of said aluminum film, and
said aluminum film is subjected to anodic oxidation to form an oxide film on its surface.
In those constitutions noted above, the aluminum film may contain impurities.
In those, the side-etched region of the titanium film is filled with the anodic oxide film of aluminum.
Still another aspect of the invention is a method for producing a semiconductor device, comprising;
a step of forming a pattern of a laminate film composed of a titanium film and an aluminum film,
a step of side-etching said titanium film,
a step of forming an oxide film on the surface of said aluminum film through anodic oxidation,
a step of forming an amorphous silicon film above said pattern, and
a step of crystallizing said amorphous silicon film under heat.
Still another aspect of the invention is a method for producing a semiconductor device, comprising;
a step of forming a pattern of a laminate film composed of a titanium film and an aluminum film,
a step of side-etching said titanium film,
a step of intentionally forming projections on the surface of said aluminum film under heat,
a step of forming an oxide film on the surface of said aluminum film through anodic oxidation,
a step of forming an amorphous silicon film above said pattern, and
a step of crystallizing said amorphous silicon film under heat.
In those constitutions, the heating may be effected in a heating furnace. It may also be attained through exposure to strong radiations.
The heating temperature is preferably not lower than 500xc2x0 C. to produce better results in the invention.
In the invention, it is desirable to use aluminum having an extremely high purity. Using aluminum having a higher purity retards the formation of hillocks and whiskers on the surface of the aluminum film.
In order to retard the formation of hillocks and whiskers, impurities may be added to the aluminum film in an amount not larger than a few % by weight. The impurities include, for example, Si, Sc, Ti and Y, and even other elements selected from lanthanides and actinides.